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An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect

An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect. Outline Physics Motivation Different methods of lifetime measurements The PrimEx experiment and our first results Control of systematic errors Summary. A. Gasparian

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An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect

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  1. An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect Outline • Physics Motivation • Different methods of lifetime measurements • The PrimEx experiment and our first results • Control of systematic errors • Summary A. Gasparian NC A&T State University, Greensboro, NC

  2. 0 decay width • 0→ decay proceeds primarily via chiral anomaly in QCD. • The prediction of chiral anomaly is exact for massless quarks: • Corrections to chiral anomaly prediction: (u-d quark masses and mass differences) Calculations in NLO ChPT: (J. Goity, at al. Phys. Rev. D66:076014, 2002) Γ(0) = 8.10eV ± 1.0% ~4% higher than LO, uncertainty less than 1% • Recent calculations in QCD sum rule: (B.L. Ioffe, at al. Phys. Lett. B647, p. 389, 2007) • Γ() is only input parameter • 0- mixing included Γ(0) = 7.93eV ± 1.5% 0→ • Precision measurements of (0→) at percent levelwill provide a stringent test of fundamental predictions of QCD. PAC33, January 15, 2008

  3. Decay Length Measurements (Direct Method) Measure 0decay length 1x10-16 sec too small to measure solution: Create energetic 0 ‘s, L = vE/m But, for E= 1000 GeV, Lmean 100 μm very challenging experiment An experiment had been done at CERN, in 1984, P=450 GeV proton beam 2 variable (5-250m) foils Result: (0) = 7.34eV3.1%(total) Dominant systematic error: Uncertainty in P0 (1.5%) • Limitations of method • unknown P0 spectrum • foil position dependent exptl. bgnd. PAC33, January 15, 2008

  4. e+e- Collider Experiment e+e-e+e-**e+e-0e+e- e+,e- scattered at small angles (not detected) only  detected experiment: DORIS II @ DESY Results: Γ(0) = (7.7 ± 0.5 ± 0.5 eV ( ± 10.0%) • dominant systematic errors: • luminosity (~6%) • beam-residual gas interaction • Not included in PDG average • Limitations of method • luminosity • unknown q2 for ** PAC33, January 15, 2008

  5. ρ,ω Primakoff Method Nucl. Coherent 12C target Primakoff Nucl. Incoh. Interference Challenge: Extract the Primakoff amplitude PAC33, January 15, 2008

  6. Previous Primakoff Experiments • DESY (1970) • bremsstrahlung  beam, E=1.5 and 2.5 GeV • Targets C, Zn, Al, Pb • Result: (0)=(11.71.2) eV 10.% • Cornell (1974) • bresstrahlung  beam E=4 and 6 GeV • targets: Be, Al, Cu, Ag, U • Result: (0)=(7.920.42) eV 5.3% • dominant systematic errors: • N (4%) and quantameter (2%) • All previous experiments used: • Bremsstrahlung (untagged)  beam • Conventional Pb-glass calorimetry PAC33, January 15, 2008

  7. PrimEx Experiment • Requirements to Setup: • high angular resolution (~0.5 mrad) • high resolutions in calorimeter • small beam spot size (‹1mm) • Background: • tagging system needed • Particle ID for (-charged part.) • veto detectors needed • JLab Hall B high resolution high intensity photon tagging facility • New pair spectrometer for photon flux control at high intensities • New high resolution hybrid multi-channel calorimeter (HYCAL) PAC33, January 15, 2008

  8. PrimEx Milestones • Proposal approved in 1999 by PAC15, re-approved by PAC22 (E02-103) in 2002 with A rating. • In 2000 NSF awarded a collaborative MRI grant of $1 M to develop the experimental setup. • Full support of JLab (Engineering group, machine-shop, installation, etc.). • In 4 years the experimental setup, including procurement of all hardware, was designed, constructed and tested. • Commissioning and data taking was performed in August-November 2004 run. • Preliminary results had been released at APS April, 2007 meeting with AIP press conference. • First publication is expected in spring, 2008. PAC33, January 15, 2008

  9. Luminosity Control: Pair Spectrometer • Precision cross section measurement: • photon flux at 1% level required • absolute tagging ratios: • TAC measurements at low intensities • relative tagging ratios: • pair spectrometer at low and high intensities Scint. Det. Dipole e+ HYCAL e- • Checked by cross sections of known EM processes at the 1% level: • Compton scattering • e+e- pair production Photon beam PAC33, January 15, 2008

  10. Electromagnetic Calorimeter: HYCAL • Energy resolution • Position resolution • Good photon detection efficiency @ 0.1 – 5 GeV; • Large geometrical acceptance PbWO4 crystals resolution Pb-glass budget Design concept hybrid calorimeter 1152 PbWO4 crystal detectors 576 Pb-glass Cherenkov detectors PAC33, January 15, 2008

  11. - Invariant Mass Resolution PAC33, January 15, 2008

  12. 0 Event selection We measure: • initial photon energy: E and time • energies of decayed photons: E1, E2 and time • X,Y positions of decayed photons Kinematical constrains: • Conservation of energy; • Conservation of momentum; • m invariant mass Three groups analyzed the data independently PAC33, January 15, 2008

  13. Differential Cross section GEANT: • acceptances; • efficiencies; • resolutions; Experimental Yield per  Diff. cross section PAC33, January 15, 2008

  14. 0 Forward Photoproduction off Complex Nuclei(theoretical models) Leading order processes: • Coherent ProductionA→0A Nuclear coherent Primakoff Next-to-leading order: Photon shadowing 0 rescattering PAC33, January 15, 2008

  15. 0 Forward Photoproduction off Complex Nuclei(theoretical models) • Two independent approaches: • Glauber theory • Cascade Model (Monte Carlo) • Incoherent Production A→0A´ Deviation in Γ(0) Extraction: less than <0.2% PAC33, January 15, 2008

  16. Fit to Extract 0 Decay Width • Theoretical angular distributions smeared with experimental resolutions are fit to the data • Combined average from three groups: Γ(0)  7.93 eV  2.10%(stat.)  2.0% (syst) PAC33, January 15, 2008

  17. PrimEx Current Result () = 7.93eV2.1%2.0% 0 Decay width (eV) ±1.% PAC33, January 15, 2008

  18. Estimated Systematic Errors PAC33, January 15, 2008

  19. Compton Cross section: Theory Pure QED process: Should be calculable on percent level • Leading Order: Klein-Nishina • Corrections to LO: • Rad. correction (virtual/soft) Double Compton (hard emiss.) • Klein-Nishina + full rad. Corr. (Monte Carlo Method) • Klein-Nishina + full rad. Corr. (Numerical Integration Method) PAC33, January 15, 2008

  20. Compton Cross section: Experiment Average stat. error: 0.6% Average syst. error: 1.2% Total: 1.3% Δσ/ΔΩ (mb/6.9 msrad) PAC33, January 15, 2008

  21. Summary • The 0 lifetime is one of the few parameter-free predictions in QCD reflecting effects of fundamental symmetry and axial anomaly. • Percent level measurement is a stringent test of QCD at these energies. • A state-of-the-art high resolution experimental setup including a high precision EM calorimeter and pair spectrometer has been designed, developed, constructed and commissioned with first physics run in fall, 2004. • Availability of high resolution and high intensity tagging facility together with recent developments in calorimetry made the Primakoff method the viable way to reach the projected percent level in 0 decay width. • Compton and pair-production cross section measurementsdemonstrate that the systematic errors are controlled at 1.3% level. • The experimental setup is capable for a percent level cross section measurement. • Control of model error in 0 lifetime at 0.25% level has been reached. • Our first result: Γ(0)  7.93 eV  2.10% (stat.)  2.0% (syst.) • Requesting 28 days of beam time to reach the goal of 1.4% on 0 life time. PAC33, January 15, 2008

  22. The End PAC33, January 15, 2008

  23. Stability of relative tagging ratios Monitored by PS during production data taking. PS+tagger Tagger PAC33, January 15, 2008

  24. 0 Event selection (cont.) Three groups analyzed the data independently PAC33, January 15, 2008

  25. Theoretical Study of0 Forward Photoproduction off Complex Nuclei Coherent ProductionA→0A: Absorption of 0 Primakoff Nuclear coherent Photon shadowing 0 rescattering PAC33, January 15, 2008

  26. Model dependence of Γ(0) Extraction Model error in Γ(0) Extraction can be controlled at < 0.25% PAC33, January 15, 2008

  27. Some results on Coherent Production A→0A • Electromagnetic form factors 12C E=5.2 GeV 208Pb • Strong form factors 208PbE=5.2 GeV Without shadowing With shadowing PAC33, January 15, 2008

  28. Incoherent ProductionA→0A´ Two independent approaches: • Glauber theory • Cascade Model Deviation in Γ(0) Extraction is <0.2% PAC33, January 15, 2008

  29. Differential Cross section GEANT: • acceptances; • efficiencies; • resolutions; Experimental Yield per  Diff. cross section PAC33, January 15, 2008

  30. New from Ilya, 011208(animation) • Theoretical angular distributions smeared with experimental resolutions are fit to the data • Combined average from three groups: Γ(0)  7.93 eV  2.10%(stat.) PAC33, January 15, 2008

  31. Control of Systematic Errors: Compton We measure: • Initial photon energy: E and time • Energies of scattered particles: E, Ee and time • X,Y positions on HYCAL Events Selection • Energy conservation • 3-momentume conservation (including co-planarity) PAC33, January 15, 2008

  32. Estimated Systematic Errors on Compton (preliminary) PAC33, January 15, 2008

  33. PrimEx Collaboration North Carolina A&T State University University of Massachusetts Idaho State University University of North Carolina Wilmington Jefferson Lab MIT Catholic University of America Arizona State University CIAE Beijing, China Norfolk State University Beijing University, China Lanzhou University, China ITEP Moscow, Russia IHEP Protvino, Russia Duke University Kharkov Inst. of Physics and Tech. Ukraine Northwestern University IHEP, China University of Sao Paulo, Brazil Yerevan Physics Institute, Armenia RIKEN, Japan JINR Dubna, Russia USTC, China Hampton University George Washington University PAC33, January 15, 2008

  34. Compton as Stability Control(maybe to question section) σ (mb) PAC33, January 15, 2008

  35. ρ, ω Primakoff Method Challenge: Extract the Primakoff amplitude PAC33, January 15, 2008

  36. Compton Cross section PAC33, January 15, 2008

  37. Trigger Improvement PAC33, January 15, 2008

  38. e+e- Pair Production in PrimEx • Agreement with theory at the level of 2.5% • Work in progress to reduce the systematic errors to 1-2% level PAC33, January 15, 2008

  39. An Example: Precision Measurement of → decay width • All  decay widths are calculated from  decay width and experimental Branching Ratios (B.R.): Γ(η→ decay) = Γ(→) × B.R. • Any improvement in Γ(→) will change the whole - sector in PDB PAC33, January 15, 2008

  40. Compton Cross section PAC33, January 15, 2008

  41. PbWO4 Development:Optical Properties Optical transparency PAC33, January 15, 2008

  42. PbWO4 Development Specified Size: 20.5x20.5x180 mm3 Tolerances: +0.0-0.1 in trans. +0.3-0.0 in long. • Collaboration managed to double the number of crystals: to from 650 to 1250 • Critical for the experiment PAC33, January 15, 2008

  43. 0 decay width: recent theoretical advances • QCD sum rule approach: • f0 - f+ caused by strong interaction shown to be small • 0 -  mixing included Γ(0) = 7.93eV ± 1.5% • error is dominated by Γ() decay width 0→ • Precision measurements of (0→) at percent levelwill provide ultimate test of fundamental predictions of QCD. PAC33, January 15, 2008

  44. (0→)World Data (do not need) • 0 is lightest quark-antiquark hadron • The lifetime:  = B.R.( 0 →γγ)/(0 →γγ) 0.8 x 10-16 second ±1% • Branching ratio: B.R. ( 0→γγ)= (98.8±0.032)% 0→ PAC33, January 15, 2008

  45. Impact of Giant Excitation of Nucleus on 0 Primakoff production • With nuclear collective excitation, the longitudinal momentum transfer in 0 photo-production is Δin= Δ+Eav, where the average excitation energy Eav for 12C is ~20-25 MeV. • The ratio of the cross section of the 0 photo-production in the Coulomb field with nuclear excitation to “elastic” electromagnetic production can be estimated as: • Nuclear Giant Excitation effect for lead is small as well. PAC33, January 15, 2008

  46. Outline • Physics Motivation • Different methods of lifetime measurements • The PrimEx experiment and our first results • Control of systematic errors • Summary

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